In an ADSL transceiver, the circuitry connecting between an integrated circuit of an analog front end (AFE) and the transmission line is referred to as an analog line interface. An AFE is a circuit block that provides the interface between the line transceiver and a digital signal processing (DSP) processor. FIG. 1 illustrates a typical hybrid structure of a conventional analog interface of an ADSL system. As shown in FIG. 1, an analog line interface typically includes a line driver 1, a hybrid circuit 3, a receive amplifier 5, and a transformer 7. The transmit signal (TX) is typically coming from a DSP portion 8 (such as transmit shaping filters) of the ADSL transceiver through a digital to analog converter (DAC) of AFE (not shown). The transmit signal is then supplied with sufficient voltage and current by the line driver 1, and coupled via the hybrid circuit 3 and the transformer 7 to a transmission line 9, such as a telephone line or twisted-pair loop (twisted-pair copper line). The transmission line 9 has a certain line impedance Z (typically 100 Ω). The receive signal (RX) from the transmission line 9 is also coupled via the transformer 7 to the transceiver. The receive signal is coupled by the hybrid circuit 3 to the receive amplifier 5, and then supplied to the DSP portion 8 (such as channel equalizers) through an analog to digital converter (ADC) of AFE (not shown).
The transformer 7 is defined to have a turns ratio of 1: n. The turns ratio of the transformer 7 can be used to provide gain to the transmitted signal, and has a major effect to the power supply voltages of the line driver amplifiers. When the turns ratio n is greater than 1, the transformer 7 performs a step-up (by factor n) of the transmit signal to the transmission line 9, and thus the amount of voltage swing needed by the line driver amplifier is reduced.
However, a step-up transformer also performs a corresponding step-down (by factor 1/n) of the receive signal received from the transmission line, reducing the receive signal amplitude and signal to noise ratio (SNR). Further attenuation of the receive signal by the transformer in addition to the inherent transmission line attenuation can cause the malfunction of the transceiver, depending on the sensitivity of the receiver circuitry. However, there are some reasons preferring a turns ratio greater than 1.
First, it is desirable to integrate the TX line driver into an AFE chip so as to minimize the cost. This normally implies that the TX line driver has to operate at a low supply voltage, for example, 5V or 3.3V. However, since the ADSL transmit signal on the line is typically required to have about 15 volts peak-to-peak differential (Vppd), it is necessary to step-up the transmit signal by a turns ratio of, for example, n=4.25.
Furthermore, it is desirable to reduce the number of different voltage supplies needed to implement a DSL transceiver. Thus, it is also desirable that the line driver operates at the same supply voltage, for example, as the AFE chip, instead of its conventional voltage supply of 10-12 V which is required to drive the transmit signal without a step-up (i.e., the turns ratio of 1).
In addition, reducing the power required for a line driver is desirable, for example, in the Universal Serial Bus (USB)-based customer premises equipment (CPE) modems, such as a modem integrated into a personal computer (PC).
As mentioned above, however, a step-up of the transmit signal means the corresponding step-down of the receive signal, which degrades the SNR of the receive signal by the same amount. Such a degraded noise performance of a line interface directly affects the data rate of the transceiver. Accordingly, it would be desirable to build a line interface that can simultaneously allow a line driver operating from a low power supply voltage and not incur the severe noise penalty associated with the traditional line interface with a hybrid circuit.